nvidia*: Bring under x11@ umbrella

This brings all nvidia ports under x11@ per some previous comments from
danfe@ and manu@.

The expectation for commits will be a simple quorum of interested people
(i.e. danfe, ashafer, kbowling) for peer review and commit.

Since the nvidia-secondary driver is chained off the main one, the
current maintainer has been notified of the change and can still
participate via freebsd-x11@ and project tools but it does not make
sense to have it differentiated.

Approved by: danfe, manu